It is known in the art to use Surface Mount Technology (SMT) to assemble integrated circuits. The performance of this technology involves various steps including, by example, the application of solder cream to a substrate, the placement of circuit components on the substrate, soldering, and cleaning. During the performance of these steps, the circuit components often need to be protected from, by example, solder, solvents, or other types of materials that can contaminate these components during the assembly process.
The need for using miniature and high performance filters in wireless communication devices has led to the widespread usage of Surface Acoustic Wave (SAW) filters. As is known by those having skill in the art, layer surfaces of SAW filters typically need to be protected from external contaminants during assembly and subsequent use. One known method of protecting these layer surfaces during assembly includes assembling the filters using, for example, flip-chip technology in a hermetic environment. As can be appreciated, this technique can be tedious to perform.
Another known method of protecting layer surfaces of SAW filters includes packaging the SAW filters in hermetically sealed ceramic packages. After being packaged in this manner, the SAW filters can then be surface mounted to a circuit board. Unfortunately, this technique tends to be expensive, as packaging costs can contribute considerably to overall fabrication costs.
Fortunately, the drawbacks associated with using hermetically sealed ceramic packaging and/or hermetic environments can be avoided by using another type of high performance acoustic filter, namely a Bulk Acoustic Wave (BAW) filter. BAW filters typically include several BAW resonators. In a BAW filter, acoustic waves propagate in a direction that is perpendicular to the filter's layer surfaces. In contrast, acoustic waves which propagate within a SAW filter do so in a direction that is parallel to the layer surfaces of the filter. As a result, and as may be appreciated by those who are skilled in the art, the performance of a SAW filter is likely to be more detrimentally effected when its layer surfaces are contaminated or otherwise impaired by external elements than would be the performance of a BAW filter that experiences similar contamination.
BAW filters can be fabricated to include various known types of BAW resonators. These known types of BAW resonators comprise three basic portions. A first one of the portions, which is used to generate acoustic waves, includes an acoustically-active piezoelectric layer. This layer may comprise, by example, zinc-oxide (ZnO), aluminum nitride (AlN), zinc-sulfur (ZnS), or any other suitable piezoelectric material that can be fabricated as a thin film. A second one of the portions includes electrodes that are formed on opposite sides of the piezoelectric layer. A third portion of the BAW resonator includes a mechanism for acoustically isolating the substrate from vibrations produced by the piezoelectric layer. BAW resonators are typically fabricated on silicon, gallium arsenide, or glass substrates using thin film technology (e.g., sputtering, chemical vapor deposition, etc.). BAW resonators exhibit series and parallel resonances that are similar to those of, by example, crystal resonators. Resonant frequencies of BAW resonators can typically range from about 0.5 GH to 5 GHz, depending on the layer thicknesses of the devices.
As can be appreciated by those who are skilled in the art, at least some of the layers that form BAW filters may be exposed to the air. If a contaminating or otherwise harmful external material comes into contact with any of these layers, the performance of the BAW filters can become degraded. In order to avoid this problem, these layers are typically protected using a semi-hermetic packaging. Unfortunately, the costs of using semi-hermetic packaging can be high. Thus, it would be desirable to provide a novel, inexpensive technique for protecting these surfaces. It would also be desirable to fabricate BAW filters in a manner so as to provide interfaces for reflecting acoustic waves that replace the layer and air interfaces of known types of BAW filters.